This invention relates to an electrical power supply for a flash unit using a direct-current high voltage electrical power source such as a layer-built cell.
An electrical power supply circuit for a flash unit using a direct-current high voltage (100-500 volts) electrical power source is conventionally constructed as shown in FIG. 1. Here, two power switches 2 and 2' when turned on allow for charging a main capacitor 5 through a resistor 3 and a discharge preventing diode 4. In such as conventional electrical power supply circuit, the electrical power source switches use mechanical contacts. It often happens that the switches 2 and 2' are opened when the main capacitor 5 is not yet charged to a sufficient level. When that occurs, the application of a high voltage across the contacts of the switches 2 and 2' may cause an aerial discharge between the contacts. Thus the electrical power source is then not actually cut off but continues charging the main capacitor 5. If the aerial discharge continues for a long time, there is a great possibility of damaging the contacts due to the heat of the discharge.
The problem has so far been solved by lengthening the distance between the contacts of the electrical power source switch so that even when a high voltage is applied across the contacts, no aerial discharge is produced. This solution gives rise to an alternate problem that the bulk and size of the electrical power source switch in the electrical power source circuit becomes much larger.